The present disclosure relates to self-aligned double patterning (SADP) for semiconductor device fabrication, and more specifically, to an SADP method using spacers having mandrel undercut spacer portions that provide dimension control for the mandrel space.
Photolithography is a technique for transferring an image rendered on one media onto another media photographically. Photolithography techniques are widely used in semiconductor fabrication. Typically, a circuit pattern is rendered as a positive or negative mask image which is then projected onto a silicon substrate coated with photosensitive materials (e.g., PR). Radiation impinges on the masked surface to chemically change those areas of the coating exposed to the radiation, usually by polymerizing the exposed coating. The un-polymerized areas are removed, being more soluble in the developer than the polymerized regions, and the desired image pattern remains.
In the microelectronics industry as well as in other industries involving construction of microscopic structures (e.g., micromachines, magnetoresistive heads, etc.) there is a continued desire to reduce the size of structural features and microelectronic devices and/or to provide a greater amount of circuitry for a given chip size. Miniaturization in general allows for increased performance (more processing per clock cycle and less heat generated) at lower power levels and lower cost. Present technology is at atomic level scaling of certain micro-devices such as logic gates. FETs and capacitors, for example. Circuit chips with hundreds of millions of such devices are common.
In order to achieve further size reductions exceeding the physical limits of trace lines and micro-devices that are embedded upon and within their semiconductor substrates, techniques that exceed lithographic capabilities have been employed. Self-aligned double patterning (SADP), also known as sidewall image transfer (SIT), is one such technique to generate sub-lithographic structures. SADP involves the usage of a sacrificial structure (e.g., a mandrel, typically composed of a polycrystalline silicon), and a sidewall spacer (such as silicon dioxide or silicon nitride, for example) having a dimension less than that permitted by the current lithographic ground rules formed on the sides of the mandrel (e.g., via oxidization or film deposition and etching). After removal of the mandrel, the remaining sidewall spacer is used as a hard mask (HM) to etch the layer(s) below, for example, with a directional reactive ion etch (RIE). Since the sidewall spacer has a sub-lithographic lateral dimension, i.e., width, (less than lithography allows), the structure formed in the layer below will also have a sub-lithographic lateral dimension.
One challenge with SADP processing is continuing to reduce and control the lateral critical dimension (CD) of the mandrel, which controls the spacing between structures formed by the sidewall spacers. The lateral CD of the mandrel size controls the spacing between sidewall spacers formed thereabout. This space between sidewall spacers formed aside the mandrel is referred to as the ‘mandrel space.’ A size of a space between sidewall spacers on adjacent mandrels is controlled by the spacing between adjacent mandrels and the thickness of the sidewall spacer material. This latter space is referred to as the ‘non-mandrel’ space. At the 7 nm technology node and beyond, controlling the mandrel's lateral CD (i.e., width) is very difficult due to, for example, photolithography limitations in printing a small mandrel, and etching limitations in reducing dimensions too much.